Reply to Family social media posts

This is exactly how my in-laws are. My sister in law goes around telling people that my son is her son and no matter how many times I talk to my in-laws and bf they don’t listen to me at all. And now I am almost 5 months pregnant and my in-laws are already claiming that my baby is there’s. My sister in law claims that as well due to all of their sibling looking alike. My son came out looking like his father who conveniently looks like his sister so now she claims any child of mine is hers. And she’ll post that on social media. All I do I get mad and talk to my BF . To the point I have told him that his side will not have any right on seeing any of my kids.